1. Message Service System
M. L. Liu
12/27/2018
Distributed Computing, M. L. Liu

2. The Message System Paradigm
The Message System or Message-Oriented Middleware (MOM) paradigm is an elaboration of the basic message-passing paradigm.
In this paradigm, a message system serves as an intermediary among separate, independent processes.
The message system acts as a switch for messages, through which processes exchange messages asynchronously, in a decoupled manner.
A sender deposits a message with the message system, which forwards it to a message queue associated with each receiver. Once a message is sent, the sender is free to move on to other tasks.
12/27/2018
Distributed Computing, M. L. Liu

3. The Message System Paradigm
12/27/2018
Distributed Computing, M. L. Liu

4. Distributed Computing, M. L. Liu
Message System Models
Two subtypes of message system models:
Point-to-point
Publish-subscribe
12/27/2018
Distributed Computing, M. L. Liu

5. The Point-To-Point Message Model
  In this model, a message system forwards a message from the sender to the receiver’s message queue. Unlike the basic message passing model, the middleware provides a message depository, and allows the sending and the receiving to be decoupled. Via the middleware, a sender deposits a message in the message queue of the receiving process. A receiving process extracts the messages from its message queue, and handles each one accordingly.
Compared to the basic message-passing model, this paradigm provides the additional abstraction for asynchronous operations.
12/27/2018
Distributed Computing, M. L. Liu

6. The Publish/Subscribe Message Model
In this model, each message is associated with a specific topic or event. Applications interested in the occurrence of a specific event may subscribe to messages for that event. When the awaited event occurs, the process publishes a message announcing the event or topic. The middleware message system distributes the message to all its subscribers.
The publish/subscribe message model offers a powerful abstraction for multicasting or group communication. The publish operation allows a process to multicast to a group of processes, and the subscribe operation allows a process to listen for such multicast.
12/27/2018
Distributed Computing, M. L. Liu

7. Toolkits based on the Message-System Paradigm
The MOM paradigm has had a long history in distributed applications.
Message Queue Services (MQS) have been in use since the 1980’s.
The IBM MQ*Series6 is an example of such a facility.
Other existing support for this paradigm are
Microsoft’s Message Queue (MSQ),
Java’s Message Service
12/27/2018
Distributed Computing, M. L. Liu

8. Sources of Information
Sun's JMS Tutorial
Microsoft's Message Queue(MSMQ)
IBM's MQ-Series
12/27/2018
Distributed Computing, M. L. Liu